Die cast waveguide low pass filter

ABSTRACT

A microwave low pass filter of the so-called &#39;&#39;&#39;&#39;waffle iron&#39;&#39;&#39;&#39; type is disclosed. The filter utilizes die cast construction techniques and cast-in dowel holes for dimensional fidelity, close tolerances and precise alignment. The filter uses fewer parts than previous devices and can be easily assembled without the use of jigs, fixtures or special bonding techniques. Die cast construction techniques are made possible in the filter design by departing from the optimum electrical design of such components.

n1] 3,777,286 14 1 Dec. 4, 1973 United States Patent v Cramm et al.

[54] I CAST WAVEGUIDE LOW PASS FILTER 3,205,462 9/1965 Meinke 333/98 M[75] Inventors: Herman W. Cramm, Rolling Hills 333/98 M 9/l966 333/73 R3,546,635 8/1968 Quine et al....

Estates; Clifford L. Gorby,

Englewood; Juri G. Leetmaa, Los Angeles, all of Calif.

Assignee: Hughes Aircraft Company, Culver Primary Examiner-Rudolph V.Rolinec Assistant Examiner-Saxfield Chatmon, Jr. Attorney- W. H.MacAllister et al.

City, Calif.

57 ABSTRACT A microwave 'low pass filter of the so-called waffle Aug. 7,1972 Appl. No.2 278,667

[22] Filed:

iron" type is disclosed. The filter utilizes die cast con- 7 structiontechniques and cast-in dowel holes for di- [52] US. 333/98 M mensionalfidelity, close tolerances and precise align- [5 l] l-l0 3h 7/10, H03h9/00 333/70 S, 333/73 W Int. ment. The filter uses fewer parts thanprevious devices [58] Field of Search................ 333/73, 73 W, 98M, and an be easily assembled without the use of jigs,

' fixtures or special bonding techniques. Die cast constructiontechniques are made possible in the filter design by departing from theoptimum electrical design of such components.

[5 6] References Cited UNITED STATES PATENTS 3,046,503 7/l962 333 73 w 6Claims, 3 Drawing Figures DIE CAST WAVEGUIDE LOW-PASS -F-lLTER iFIELD'OF. THE INVENTION This invention relates to microwavefilters-andmorev specifically to microwavelow pass filters of-the waffl'e" irontype.

DESCRlPTlON OF' THE PRlOR 'ART' In the past, the more; complex'microwavedevices have been fabricated according to the time-honoredtraditions'of-the precision machine shop: Certainly,

with the more complex-and. physically intricate structures the-onlypractical way to obtain devices of the required dimensional. toleranceshas been by precision machining. According .to these-techniqueswaffleiron filters have beencassembled from no fewer 'than four separatepieces, each havingibeen-"previously machined.

of block stock brass or other material;

As is well-known, microwave filtersof the'waffle'iron type generallycomprise two identical sections of block" material, each. having:transverse and longitudinal rooves machined therein tocreatea'm'atrix ofraised bosses or teeth. These two sections of raised teeth are thenaligned and=assembled so-that the=teeth-of each section are identicallyaligned withth'ose of th'e other sections in the manner ofa householdwaffle iron.

Apair of sidewallsare provided to createa hollow conductively boundedstructure.- Typicalstructures of the prior art type-are illustratedin;U.S."Pat."No.3,046-,503

which issued on July 24,- 1962 to S.'B; Cohn.

In addition to the waffl e'iron'sections and conductive sidewalls, otherparts, including=.impedance-matching'- sections and endflanges arerequiredfor the complete filter structure.-

The main source of difficulty'withsuch devices ori'giynates in-theassembly PI'OCCSSuNOt only must'each sepa-' rate waffle ironsectionbe-checked for dimensional ac curacybut also they-must beprecisely alignedat the time of assembly. If the teethofthe-assembledstructure are not precisely -.aligned,'especially in thetransversedirectionundersirable spurious modes are generatedduring theoperation-of the device: These spurious modes in turn causeseveredegradation of the electri-- cal performance of the filter.

Proposals to modifythe basic'waffl'e' iron design to attenuate thespurious modes which are generated have' been made.Such-modified'structures are'disclosed in US. Pat. No. 3,271,706 whichissued on Sept. 6, 1966 to]. P. Rooney. Such remedies, however, donotpre-' vent the-generation of spurious modes, butrather tend" toeliminate them only after they are generated. Fur

thermore, the'modified structures tendto be somewhat larger and morecomplicated than the basic-filter.

It is an object of the-present invention toprovide a microwave waffleiron filter 'of simplified construction:

2fmodifyingtheiphysical design'vof thevdie cast part. The difficulty inso doing: results' from the fact well-known in the microwave deviceartthat a modification of the physical design invariably results in amodification of the electrical-perfonnancee Theproblem, therefore,

becomes one of designinga microwave component not only from"considerations of *electricaland mechanical behavior but also byconsiderationsdictated by the die castingaart.

It is another-object of the present invention, therefore, to modify-thedesign'of waffle iron filters to accommodate diecastfabrication'techniques without adversely affecting their electricalproperties.

SUMMARY OF THE INVENTION In keeping-with the'principles of the presentinvention,*- th'e"se objects are'accomplished by modifying the design ofthe waffleironfilter to accommodate die-cast construction techniques.Such'modifications for example; includ'split sidewalls'with'integralflanges having cast-in dowel pin'aligning holes and screw'h'oles.Additionally, the-design departs from the previously employedrectangular waveguide cross-sections and rectangularteeth byincorporating taperedwaveguide walls and taper'edsteethaAlthoughthe'modifications in design depart from the'theoretically optimum design,they result in'substantially no sacrifice in electrical performance.

In addition, die casbconstruction techniques providemuchff'lowerm'anufacturingi-costs, improved dimensional'fidelity,closetolerances, fine'det'ail," smooth finish'esy-little'ornomachin'ing'for'assembly sets and exact part to-part"duplication; all'ofwhichj'are required to produce high quality waffle iron filters.-

It is another object ofthepresent"inventiont0provvidewaffle--ironfilters' havingfewer constituent parts:

Althoughit was proposed as -early as 4948mm certain microwave componentswere susceptible to die cast fabrication,- these-methods have not been'widelyemployedzDie casting ,as properly employed is known to yieldcastings of clean, sharp'definition and close dimensionaltolerances-(see:Ragam-Microwave Transmis-- sian Circuits, Vol. 9. RadnLab." Series, McGraw-hill,

New York, l948, -p. 122).

The practical problems associated-with 'such methods, howevenfrequentlycan be overcome only by BRIEF-DESCRIPTION OF THE DRAWINGSTheabovemientionedandother features and objects of the present inventionwill become more apparent by reference -to 'thefollowing descriptiontaken in con- 'junction"-with*the' accompanying drawings in which:

FlG. 1 is apictorialview of one of the mating halves of awaffleiron'filter accordingto the present 'inventron;

FIG. 2 isa-longitudinalcross-sectionof the assembled'filter in'somewhatmodified form; and

FIG. 3 is an=end view of the assembled filter particularlyillustratingthe' non-rectangular waveguide crosssectionx DESCRIPTION OFTHE PREFERRED EMBODIMENTS Referring'more specifically to'the drawings,FIG. 1 is a pictorial view *of' oneof the mating halves of a preferredembodiment of the present invention. The part flanges l4 and 15 arelapped, machined or sanded to a smooth flat finish and preferably platedprior to assembly to provide a high conductivity substantially loss-freejoint. To facilitate alignment of body with its mating upper half, dowelpin alignment holes 16 are provided in flanges 14 and 15. Tapped screwholes 17 also extend through these flanges to receive machine screwswhich provide the necessary clamping force to maintain the integrity ofthe assembled structure.

The internal construction of the waffle iron filter of the presentinvention can be readily seen in the pictorial view of FIG. 1. Again itis to be noted that an identical mating half is required to complete theassembled filter. The die cast body of FIG. 1 comprises three seriallydisposed sections. Proceeding from waveguide flange 11 there is shown aninput impedance matching section 18, a centrally disposed filter section19 and an output impedance matching section 20. The design of the waffleiron section 19 and impedance matching sections 18 and 20 are well-knownin the art. See, for example, US. Pat. No. 3,046,503 mentionedhereinabove, and an article by Young and Shiffman entitled New andImproved Types of Waffle-Iron Filters, Proc. IEE (London), Vol. 110, pp.1191 et seq., July 1963.

For the purpose of facilitating die cast construction techniques, thedesign of the waffle iron filter of the present invention departs fromthe design dictated by the prior art and by traditional microwaveengineering practice.

A taper is employed in the waveguide sidewalls as indicated by the anglea. The taper is sufficiently large to allow the die to be removed fromthe body 10 during the casting process without surface abrasion. Thetaper (usually referred to as draft) of the waveguide wall, of course,results in a non-rectangular waveguide crosssection. By utilizing ataper or draft angle a on the order of from one-half to two degrees, ithas been found that a part such as body 10 can be die cast of zincalloy, for example, with no discernable surface irregumatching section18 broken away. FIGS. 2 and 3 taken together illustrate thefrustro-pyramidal shaped teeth of 29 and 29' of the lower and uppermating halves of the improved waffle .iron filter. In addition, theirregular hexagon cross-section of the waveguide formed by bodies 10 and11 is illustrated in FIG. 3.

In addition to the draft or taper of the waveguide sidewalls and teeth29 and 29 a similar taper has been designed into the integral flangesll, 12 and 13 and 11', 12 and 13'. These tapers, although not affectingthe electrical performance, are designed into the body to facilitate thedie casting fabrication technique.

Although many different materials including alloys of aluminum, copper,lead, magnesium and tin can be successfully die cast, the most popularand widely used alloys are the alloys of zinc. A zinc alloy which hasbeen successfully employed in the fabrication of the filters of thepresent invention is known in the trade as ZAMAK No. 3 (or alternativelyZinc Alloy No. 3).

Zinc Alloy No. 3 has the following composition:

Aluminum 3.5 4.3 percent Copper 0.25 percent maximum Magnesium 0.02 .05percent Iron 0.10 percent Lead 0.005 percent maximum Cadmium 0.004percent maximum Tin 0.003 percent maximum Zinc remainder It should benoted that the range of values one-half to two percent given for thetaper angle 0: applies to magnesium and zinc alloys. In general, if thebody is to be die cast of aluminum the taper angles should be increasedto a range of approximately two to four degrees.

After casting the parts are first trimmed to remove V the burrs andrunners and are then sanded or machined larities caused by die removal.Additionally, it has been found that such a small departure from theoptimum rectangular waveguide cross-section has no discernable adverseeffect upon the electrical properties of the filter structure. Ineffect, the taper is so small that the resulting waveguide, although notof rectangular crossmodified tooth construction is illustrated in theviews of FIGS. 2 and 3. Where appropriate, like reference numerals havebeen carried over from FIG. 1 to designate like structural elements. Inaddition, the primed reference numerals designate the correspondingelements of the mated upper half of the assembled filter structure.

FIG. .2 is a longitudinal cross-sectional view of as assembled waffleiron filter according to the present invention. In FIG. 3 there is shownan end view of the assembled filter with a portion of the inputimpedance to obtain the desired flatness of the mating surfaces. Thefinished castings are then plated with a high conductivity material suchas silver. by processes wellknown in the art. The bodies are then matedwith the aid of steel dowel pins and machine screws as shown in FIGS. 2and 3.

In all cases it is understood that the above-described embodiments aremerely illustrative of but a small number of the many possible specificembodiments which can represent applications of the principles of thepresent invention. Numerous and varied other arrangements can be readilydevices in accordance with these principles by those skilled in the artwithout departing from the spirit and-scope of the invention.

What is claimed is:

1. An improved microwave filter of the type wherein a hollowconductively bounded waveguide section is provided with a plurality ofspaced conductive teeth extending from at least one wall thereof andwherein said teeth are arranged in rows and columns which are paralleland transverse to the longitudinal axis of said waveguide section,respectively, the improvement comprising:

a waveguide cross-section which is in the shape of an waveguide section,respectively, the improvement comprising:

a waveguide cross-section which is in the shape of an irregular hexagon,and wherein said teeth are in the shape of frustra of right rectangularpyramids.

3. An improved microwave filter of the type wherein a hollowconductively bounded waveguide section is provided with a firstplurality of spaced conductive teeth extending from a first wall thereofand a second plurality of spaced conductive teeth extending from a wallopposite said first wall, said first and second pluralities of saidconductive teeth being arranged in rows and columns which are paralleland transverse to the longitudinal axis of said waveguide section,respectively, the improvement comprising:

a waveguide cross-section which is in the shape of an irregular hexagon.

4. An improved microwave filter of the type wherein a hollowconductively bounded waveguide section is provided with a firstplurality of spaced conductive teeth extending from a first'wall thereofand a second plurality of spaced conductive teeth extending from a wallopposite said first wall, said first and second pluralities of saidconductive teeth being arranged in rows and columns which are paralleland transverse to the longitudinal axis of said waveguide section,respectively, the improvement comprising:

a waveguide cross-section which is in the shape of an irregular hexagon,and wherein said conductive teeth are in the shape of frustra of rightrectangular pyramids.

5. An improved microwave filter of the type wherein a hollowconductively bounded waveguide section is provided with a firstplurality of spaced conductive teeth extending from a first wall thereofand a second plurality of spaced conductive teeth extending from a wallopposite said first wall, said first and second pluralities ofconductive teeth being arranged in rows and columns which are paralleland transverse to the longitudinal axis of said waveguide section,respectively, and

wherein said waveguide section includes first and second steppedimpedance matching sections longitudinally disposed on either side ofsaid pluralities of conductive teeth, the improvement comprising:

a waveguide cross-section which is in the shape of an irregular hexagon.6. An improved microwave filter of the type wherein a hollowconductively bounded waveguide section is provided with a firstplurality of spaced conductive teeth extending from a first wall thereofand a second plurality of spaced conductive teeth extending from a wallopposite said first wall, said first and second pluralities ofconductive teeth being arranged in rows and columns which are paralleland transverse to the longitudinal axis of said waveguide section,respectively, and wherein said waveguide section includes first andsecond stepped impedance matching sections longitudinally disposed oneither side of said pluralities of conductive teeth, the improvementcomprising:

a waveguide cross-section which is in the shape of an irregular hexagon,and wherein said conductive teeth are in the shape of frustra of rightrectangular pyramids.

1. An improved microwave filter of the type wherein a hollowconductively bounded waveguide section is provided with a plurality ofspaced conductive teeth extending from at least one wall thereof andwherein said teeth are arranged in rows and columns which are paralleland transverse to the longitudinal axis of said waveguide section,respectively, the improvement comprising: a waveguide cross-sectionwhich is in the shape of an irregular hexagon.
 2. An improved microwavefilter of the type wherein a hollow conductively bounded waveguidesection is provided with a plurality of spaced conductive teethextending from at least one wall thereof and wherein said teeth arearranged in rows and columns which are parallel and transverse to thelongitudinal axis of said waveguide section, respectively, theimprovement comprising: a waveguide cross-section which is in the shapeof an irregular hexagon, and wherein said teeth are in the shape offrustra of right rectangular pyramids.
 3. An improved microwave filterof the type wherein a hollow conductively bounded waveguide section isprovided with a first plurality of spaced conductive teeth extendingfrom a first wall thereof and a second plurality of spaced conductiveteeth extending from a wall opposite said first wall, said first andsecond pluralities of said conductive teeth being arranged in rows andcolumns which are parallel and transverse to the longitudinal axis ofsaid waveguide section, respectively, the improvement comprising: awaveguide cross-section which is in the shape of an irregular hexagon.4. An improved microwave filter of the type wherein a hollowconductively bounded waveguide section is provided with a firstplurality of spaced conductive teeth extending from a first wall thereofand a second plurality of spaced conductive teeth extending from a wallopposite said first wall, said first and second pluralities of saidconductive teeth being arranged in rows and columns which are paralleland transverse to the longitudinal axis of said waveguide section,respectively, the improvemEnt comprising: a waveguide cross-sectionwhich is in the shape of an irregular hexagon, and wherein saidconductive teeth are in the shape of frustra of right rectangularpyramids.
 5. An improved microwave filter of the type wherein a hollowconductively bounded waveguide section is provided with a firstplurality of spaced conductive teeth extending from a first wall thereofand a second plurality of spaced conductive teeth extending from a wallopposite said first wall, said first and second pluralities ofconductive teeth being arranged in rows and columns which are paralleland transverse to the longitudinal axis of said waveguide section,respectively, and wherein said waveguide section includes first andsecond stepped impedance matching sections longitudinally disposed oneither side of said pluralities of conductive teeth, the improvementcomprising: a waveguide cross-section which is in the shape of anirregular hexagon.
 6. An improved microwave filter of the type wherein ahollow conductively bounded waveguide section is provided with a firstplurality of spaced conductive teeth extending from a first wall thereofand a second plurality of spaced conductive teeth extending from a wallopposite said first wall, said first and second pluralities ofconductive teeth being arranged in rows and columns which are paralleland transverse to the longitudinal axis of said waveguide section,respectively, and wherein said waveguide section includes first andsecond stepped impedance matching sections longitudinally disposed oneither side of said pluralities of conductive teeth, the improvementcomprising: a waveguide cross-section which is in the shape of anirregular hexagon, and wherein said conductive teeth are in the shape offrustra of right rectangular pyramids.